The present study was carried out to elucidate whether there are differences between the renal and fecal iodine patterns over 12-hour day and night intervals. Normal rats fed 125I-LID ad libitum and thyroidectomized animals maintained with 125I-labeled l-thyroxine showed diurnal changes in urinary and fecal iodine excretion, the highest values corresponding to the night period. The urinary volume and fecal weight were also increased during darkness hours. A positive correlation was found between the urinary loss of iodine and the quantitative iodine intake during any urine collection period. The changes of periodicity and timing of food intake alter the diurnal pattern of iodine metabolism. These observations suggest that the difference in food iodine intake between both periods constitutes the best explanation for the spontaneous cyclicity of urinary iodine excretion. However, as a significant difference in urinary iodine content exists between day and night periods in rats fed with the same amount of food within each period, and the morning corticosterone injection increases the renal loss of iodine in this period, it would indicate that other influences may be operative in regulating the diurnal variation of iodine excretion. In conclusion, the present results indicate that the high extrathyroidal metabolism of thyroid hormone and the high proportion of the hormone excreted into the feces during night hours may trigger off the TSH secretion in these situations, where an extrathyroidal pool of thyroid hormones were just adequate for normal peripheral requirements.